This invention relates to an electronic musical instrument, and more particularly to a keying system for an electronic musical instrument for gating musical tone signals in accordance with depression and release of keys in a keyboard or keyboards.
In general, the keying systems for an electronic musical instrument are classified into a direct keying system and an indirect keying system. In the indirect keying system, an envelope generator is driven by a key-ON signal (FIG. 1) which starts at the key depression and ceases at the key release, to generate an envelope waveform (FIG. 2) which rises and falls with certain time constants and in synchronism with the key-ON signal. The envelope waveform controls the operation of a gate circuit to which musical tone signals are applied, to transmit the tone signal or signals to a sound system in accordance with the envelope waveform. On the other hand, in the direct keying system, the operation of a gate circuit is directly controlled by the key-ON signal, or musical tone signals are applied directly to mechanical key switches to perform the gating of the musical tone signals. In such direct keying system, the musical tone signal begins to be outputted with its full amplitude given at the instant that the key-ON signal rises and ceases at the fall of the key-ON signal with the full amplitude at that instant, since the key-ON signal abruptly rises and falls. As seen in FIG. 3, the amplitude of the musical tone signal is not always around a "0" level at the rise and fall of the key-ON signal. Therefore, if the gating of the musical tone signal is effected by the key-ON signal when the musical tone signal is relatively large then the waveform of the gated output is allowed to rise abruptly from the "0" level to the correspondingly high level and to fall abruptly from the high level to the "0" level as indicated by the dotted line in FIG. 3, as a result of which an unpleasant click noise is generated.